The present invention relates to offshore installations including an oil/gas flowline umbilical installed between a platformxe2x80x94or a shore installationxe2x80x94and a sea bed based template/far end and including an insulated electrical conductor connected in series with the steel oil/gas pipe to a power supply.
A technical problem with such installations is to provide reliable heating means for preventing/reducing the risk of blocking and/or narrowing oil/gas flowlines when the fluid gets below a critical temperature. A solution to this problem is presented in U.S. Pat. No. 3,975,617. This patent suggests several solutions, one of which makes use of an insulated electrical conductor wound around (and connected in series with) a steel oil pipe. This series connection is terminated at a power supply.
The xe2x80x98Pipe heating system by AC in steelxe2x80x99 disclosed in said US patent depends on skin and proximity effect, hence not requiring any electrical insulation. End-connections and terminations are not described. The document does not give any hint as to whether the system was meant for on- or offshore use. If this system is installed offshore, the sea will make a part of the electric circuit since the sea is very conductive. Hence the return current will be split between the pipe/flowline and the sea, making these as a parallel connection. All the current will therefore not go in the pipe. As described in the patent, there is a skin effect that makes the current go in the outer layer of the conductor. This is however only partly true as this effect depends on both the frequency of the power source and the relative permeability of the conductor. The skin effect is given by the formula:   δ  =            2                        ωμμ          0                ⁢        σ            
where:
xcex4=Depth of penetration (the smaller the penetration, the larger is the skin effect)
xcfx89=2xcfx80ƒ
ƒ=The frequency of the current, here 60 Hz
xcexc0=Permeability of free space: 4xc3x97xcfx80xc3x9710xe2x88x927 [Wb/Axc3x97m]
xcexc=Relative permeability
"sgr"=Conductivity.
For some materials and frequencies, there is no skin effect. Due to the proximity effect, as described in said US patent, the current in the return conductor (which consists of pipe and seawater) will seek to go as close as possible to the conductor which transmits the forward current. As the system described is not electrically insulated, in addition to it being connected to ground at several places using conventional sacrificial anodes (installed for conventional corrosion protection) there is one conductorxe2x80x94i.e. the seaxe2x80x94which is closer to the cable than the pipe. The proximity effect described for the pipe, in the above patent, will also be valid for/in the sea. Hence, most of the part of the current that runs in the sea, will run close to the cable, i.e. the current density will be highest close to the cable for both the pipe and the sea, but the pipe will not be the media which transmits all the current. The pipe transmits between 50 to 70% of the total return current, while the rest is transmitted in the sea. This relationship depends on the pipe material, but mostly on the distance between the cable and pipe. Hence the system as described is not a system which consists of only cable(s) and pipe as the only conductor. If the system were installed onshore, the same current split will occur. Parts of the return current will go in the soil (which conducts relatively well), and parts will go in the pipe.
The main object of the present invention is to present a flowline umbilical which is not based on skin effect technology and which does not require complicated insulation arrangement. A further object of the invention is to avoid use of chemicals for preventing blocking and/or narrowing of the flowline.
The main features of the invention are defined in the accompanying claims. The pipeline is insulated from the platform and grounded at the far end. This new heated flowline umbilical installation shall:
prevent hydrates/wax during shutdowns and be a part of the subsea control system for template (satellite)
maintain a steady state production temperature for the pipeline above critical temperatures for wax and/or hydrate formation during shutdowns; typical temperatures may be 25xc2x0 C. for hydrate, and 33xc2x0 C. for wax
raise the temperature on pipeline from seabed temperature to required temperature after a shutdown
integrate pipeline, control umbilical and a closed direct electric heating system in one bundle.
The closed direct electric heating system within the bundle is using the pipeline as a conductor to perform heating and it is connectedxe2x80x94using a cablexe2x80x94to each end of the pipeline. The thermal insulation around the pipeline also serves as electrical insulating material to the voltage applied to the pipeline.
Available power is defined by the client at the offshore facility and required for the heating system is given by temperature requirements. The thermal insulation around the pipeline is designed with a thickness and properties which also allow for electrical insulation. This means that the thermal insulation has a thickness which is adjusted to the available power supply at the offshore facility, and the required temperature in the flowline.